To avoid damage and prevent death, most modern electrical distribution systems are grounded. Grounded systems (also known as “earthed” systems), consist of an earth-referenced grounding conductor (e.g. the third prong on a small-scale electrical outlet), which is used as a return path to enable a breaker trip during an inadvertent short-circuit condition.
Ungrounded systems, on the other hand, operate without an earth-referenced grounded conductor, so a breaker will not trip during a ground fault short-circuit condition. Since a breaker does not trip, ideal safety does not exist to protect personnel or equipment.
When is an ungrounded system necessary?
The perils of grounding an electrical system outweighs the benefits of grounding in certain situations. For example, in hospital operating rooms, it would be unacceptable to have a power outage when performing life-saving surgery. But a loss of power could occur with a “grounded” operating room. With the ungrounded power system philosophy, the electrical equipment will continue to operate even if a single ground-fault short-circuit condition occurs. When designed properly, an alarm annunciation notifies qualified electrical personnel, so they can take necessary corrective action after the surgeons have completed surgery. The benefits to continue critical operations, even when the electrical systems would otherwise want to “trip”, is obvious. But, such a system has to be designed properly.
This same ungrounded philosophy applies in other environments, where the loss of electricity is unacceptable. Steel plants during the ore melting process, intelligence facilities closing in on a capture, and data centers hosting critical information, come to mind.
How to detect a ground fault condition in an ungrounded system?
During a ground fault condition in an ungrounded system, a peripheral phase-to-ground voltage displacement occurs. This displacement is referenced with respect to ground and it does not affect equipment operation. Using the voltage displacement principle, the standard method of ground fault detection utilizes wye/broken-delta PT sensing instruments with indicator lights, voltmeters, and voltage-sensitive relays. When a single-phase experiences a ground fault condition, its own phase-to-ground voltage approaches zero and the phase-to-ground voltages of the other phases rise. With this approach, a ground fault occurrence can be detected together with identifying the specific faulted phase. However, the short-circuit could have occurred anywhere on the electrical distribution system and manually locating this ground fault, especially on a long circuit length, can cost the facility time and money.
How to locate a ground fault in an ungrounded system?
The standard method of manually de-energizing each service feeder, one at a time, and testing each load individually for fault identification, is an exhaustive and time-consuming process. A critical hospital facility cannot afford to be out of service for a prolonged duration. By incorporating a specialized signal source modulation system, using resistors and a thumper circuit into the design, a pulsing system can be used for fault location. Under this method, modulated signals travels along the faulted path and the actual fault point can be identified quickly by a qualified electrician.
When properly designed by a Professional Engineer, ungrounded systems provide continuity of operations in the event of a ground fault. Whenever ungrounded systems are used in a facility, Master Electricians need to be specially trained on how to detect and troubleshoot a ground fault on an ungrounded system.
If you would like to know if your critical facility can benefit from an ungrounded system, feel free to call me at (240) 582-3900 — Priyan De Silva, Helios Electric LLC.